Field of the Invention
This invention is in the field of viscometers that measure viscosity of liquids utilizing a flow-through type viscosity sensor.
State of the Art
Viscosity is a measure of resistance of liquid to flow and its value depends on the rate of deformation for Non-Newtonian liquids as described in Dynamics of Polymeric Liquids, Vol. 1, 1987, authored by R. B. Bird, R. C. Armstrong, and O. Hassager. The rate of deformation is given by a shear rate in a unit of (time)−1. The viscosity measured at a known shear rate is “true” viscosity. The dependence of the true viscosity on shear rate is a viscosity curve which characterizes material and is an important factor to consider for efficient processing. However, in many cases, viscosity is measured under ill-defined test condition so that shear rate cannot be known or calculated. Under ill-defined conditions, the measured viscosity value is only “apparent”. Since the true viscosity is measured at a known shear rate, the true viscosity is universal whereas the apparent viscosity is not. Instead, the apparent viscosity depends on the measuring system. For example, as a common practice, a torque of a spindle immersed in a sea of test liquid is measured while the spindle is being rotated at a constant speed. In this case the torque value only yields an apparent viscosity since the test condition is ill-defined and a shear rate is not known. At best, the apparent viscosity can be measured as a function of the rotational speed of the spindle. The rotational speed of the spindle can be in fact correlated with the shear rate only if a “constitutive equation” for the test liquid is known. However, a “constitutive equation” is not known for almost all Non-Newtonian liquids. Therefore, true viscosity can not be measured with ill-defined test conditions for most non-Newtonian liquids.
Methods of viscosity measurement that give only apparent viscosities have been developed and used for quality controls in manufacturing and material characterization. Various on-line viscometers are designed for real time viscosity measurement. U.S. Pat. No. 5,317,908 (Fitzgerald et al.) and U.S. Pat. No. 4,878,378 (Harada) are concerned with systems that measure apparent viscosities for process controls. U.S. Pat. No. 6,393,898 (Hajduk et al.) describes a system that measures many test liquids simultaneously. These viscometers measure apparent viscosities. However, because of the non-universality of the apparent viscosity measurement, a correlation of the apparent viscosity of a specific sample measured with a specific method with the true viscosity has to be found separately when desired. Fundamental development of formulations for materials requires the true viscosity measurement. Also the designs of processing equipments and accessories, such as dies, molds, extrusion screws, etc., require the true viscosity of the materials. However, the apparent viscosity measurement has been used for a quick test as an indication since it is easier and faster to measure and often more economical. The true viscosity is more difficult to get and can be only measured with a few types of instruments: rheometers and capillary viscometers. The rheometers impose a precise and known shear rate on test samples, thereby measuring true viscosities. The rheometers are versatile and usually equipped to also measure other properties. Therefore they are usually expensive. Further, large amounts of samples are usually required for viscosity measurement with a rheometer. Also, rheometers are not well suited for on-line applications. Circular capillary viscometers can measure apparent and true viscosities depending on whether a proper compensation is taken into account. The capillary viscometer needs a pressure drop measurement along the capillary for viscosity. Since the capillary is circular in cross-section, only pressure at the entrance and exit can be measured. Because of this limitation, the capillary viscometer measures only apparent viscosity unless the entrance effect is corrected for by using two different capillaries with different length to diameter ratios. However, use of two capillaries makes the capillary viscometers bulky and/or time consuming. Capillary viscometers are described in U.S. Pat. No. 6,575,019 (Larson); U.S. Pat. No. 4,920,787 (Dual et al.); U.S. Pat. No. 4,916,678 (Johnson et al.); and U.S. Pat. No. 4,793,174 (Yau). Microfluidic viscometers are disclosed in U.S. Pat. No. 6,681,616 (Michael Spaid et al.) and Publication No. 2003/0182991 (Michael Spaid et al.). Residence time of a marker in a fluidic channel is used to measure the viscosity, which is not a true viscosity unless the test liquid is Newtonian. Only an apparent viscosity is measured for non-Newtonian liquids. The portable viscometer disclosed in U.S. Pat. No. 5,503,003 (Brookfield) utilizes a well known torque measurement of a spindle rotating in a sea of liquid for viscosity measurement. As indicated, and as is well known, this method only measures apparent viscosity.
In summary, most viscosity measurement techniques yield apparent viscosity and require relatively large volumes of sample. Also, these instruments require cleaning of the parts in contact with liquid (container, spindle, etc.) before the measurement of the next sample. Such a cleaning is time consuming so that viscosity measurements typically take about 30 minutes from the set-up to the test. The larger sample volume requirement with current techniques also increases the cleaning time and waste. Therefore, there is no genuine portable viscometer which measures true viscosity for samples in small quantity and in a fast manner. The slit viscometer disclosed in my U.S. Pat. No. 7,290,441 makes it possible to measure the true viscosity of small samples. It requires, however, a precision liquid dispensing system and associated electronics to provide and control the flow of liquid through the viscometer. A simple precision liquid dispensing system which is portable and can be use with a variety of samples is not disclosed in the prior art.